1. Field of the Invention
Conventionally, it is known to remelt the surface of a workpiece, such as, for example, a cam of a camshaft for driving valves of an automotive engine, for forming a chilled layer in the surface of the workpiece so as to harden the surface. For remelting the surface of the workpiece to thereby form a molten metal layer in the surface, a beam containing heat energy is directed toward the surface. The energy beam is moved entirely or partially over the surface of the workpiece so as to form a molten metal layer on the surface. Specifically the energy beam is oscillated, or reciprocally moved, over the surface, while the workpiece, such as a cam of a camshaft, rotates about its axis of rotation so as to remelt a desired area of the surface of the workpiece, thereby forming a molten metal layer on the surface. The molten metal layer is cooled and hardened, or chilled, with time. As a result, the surface is formed with a hardened, or chilled, layer.
If a thicker chilled layer is to be formed, it is necessary for the energy beam to deliver a high level of heat energy to the surface. However, as the level of heat energy becomes higher, the time required for the molten metal layer to be cooled, or chilled, so as to solidify, becomes longer. Furthermore, because of the effect of gravity and/or of rotation of the workpiece, the molten metal tends to flow before completely solidifying. This results in a non-uniform chilled layer thickness on the surface of the workpiece.
2. Description of Related Art
To provide a uniform chilled layer thickness, an apparatus, such as that which is known from, for example, Japanese Unexamined Patent Publication No. - 60 258421, includes a plasma torch as means for generating a beam containing heat energy, which oscillates, or reciprocally moves, over the surface of the workpiece. A magnetic coil, oriented by the plasma torch, generates a magnetic field in order to cause the energy beam to oscillate between the extreme ends of the molten metal layer to be formed. The energy beam, when the plasma torch reverses the direction of movement at the extreme ends, reduces its speed and stops momentarily, so as to distribute higher heat energy to marginal portions of the surface than to the central portion. Because of this non-uniform distribution of heat energy, the chilled layer formed in the surface of the workpiece is non-uniform in its widthwise thickness. More specifically, the chilled layer is thicker, or deeper, at the opposite marginal portions than at the central portion.
To illustrate this problem more clearly, reference is made to FIG. 1, where the distribution of depth, or thickness, in a transverse direction of a chilled layer, formed on a cam surface of a camshaft by the conventional remelting and hardening treatment, is shown. A chilled layer 3 of the cam surface 2 of the cam 1 has a depth which is deeper at opposite extreme end portions 4, where the energy beam reverses its direction of movement, than at the central portion 6.